The Shoulder Complex

• Study notes on the shoulder complex

Introduction to the Shoulder Complex

• The shoulder complex includes the following joints:
  • Acromioclavicular joint
  • Sternoclavicular joint
  • Glenohumeral joint
  • Scapulothoracic joint

Objectives for studying the shoulder complex

• Discussion of the joints
• Review of the structure of the joints
• Discussion of the passive and active components
• Description of the kinematics & arthrokinematics
• Description of the integrated function of the shoulder complex

The Shoulder Complex

• Comprises 4 mechanically interrelated articulations
• Includes the sternum, clavicle, ribs, scapula, & humerus
• Engineered for mobility
• Passive structures offer limited major stability
• Mainly relies on dynamic stability
• Muscular control provides stability during active movements
• Muscle forces serve as the primary mechanism to secure the shoulder girdle to the thorax

Joints of the Shoulder Complex

• Sternoclavicular (SC) Joint
• Acromioclavicular (AC) Joint
• Scapulothoracic (ST) "Joint"
• Glenohumeral (GH) Joint

Sternoclavicular (SC) Joint

• This joint serves as the only structural attachment between the axial skeleton and the shoulder/upper extremity
• It links the medial clavicle with the manubrium of the sternum and the 1st costal cartilage
• The SC joint is a synovial, saddle joint
• Features a wedge-shaped joint space with a superiorly open orientation at rest

Osteokinematics of SC Joint

• Has 3 rotational degrees of freedom (dof)
  • Elevation/depression of the clavicle
  • Protraction/retraction of the clavicle
  • Anterior/posterior rotation of the clavicle
  • Long-axis rolling motions of the entire clavicle
• Has 3 translatory degrees of freedom (dof)
  • Magnitude is very small in a healthy joint
• Elevation & depression occur near the frontal plane
• Protraction & retraction occur near transverse plane
• Rotation occurs around longitudinal axis
• Clavicular Motions:
  • Elevation ROM: up to 48° (full range of elevation not typically used with functional arm elevation)
  • Depression ROM from neutral: less than 15°
  • Protraction ROM: 15° - 20°
  • Retraction ROM: ~ 30°
  • Anterior rotation past neutral: less than 10°
  • Posterior rotation: up to 50°

SC disc

• The SC disc operates as a pivot point for the medial end of the clavicle during movements
• It divides the joint into 2 cavities
• LImits medial translation of the clavicle
• Improves joint stability
• Increases congruence & absorbs forces transmitted along the clavicle

Sternoclavicular Capsule & Ligaments

• Relatively strong fibrous capsule
• Supported by 3 ligament complexes:
  • Anterior & posterior sternoclavicular ligaments
  • Bilaminar costoclavicular ligament
  • Interclavicular ligament
• Thick posterior capsule acts as primary restraint to anterior & posterior clavicular translations
• Anterior & posterior sternoclavicular ligaments reinforce the capsule and limit anterior & posterior translation of the medial clavicle
• Costoclavicular Ligament: very strong ligament composed of 2 bundles that limit clavicle elevation. The posterior bundle also resists medial translation of clavicle. Serves as the functional axis of rotation. Absorbs & transmits superiorly directed forces applies to the clavicle.
• Interclavicular ligament limits excessive depression of clavicle, protects brachial plexus & subclavian artery, and limits superior gliding of medial clavicle on manubrium.

Arthrokinematics of the SC Joint

• Clavicular elevation:
  • Lateral end of clavicle moves superiorly
  • Medial clavicle surface rolls superiorly & slides inferiorly on sternum and 1st rib
• Clavicular depression:
  • Lateral clavicle moves inferiorly
  • Medial clavicle surface rolls inferiorly & slides superiorly
• Clavicular retraction:
• Lateral clavicle moves posteriorly
• Medial clavicle rolls & slides posteriorly on sternum and 1st costal cartilage
• Clavicular protraction:
  • Lateral clavicle moves anteriorly in the transverse plane
  • Medial clavicle rolls & slides anteriorly on sternum and 1st costal cartilage
• Clavicular Rotation:
  • It occurs as a spin between the joint surfaces & disc
  • Clavicle rotates primarily posteriorly from neutral

Acromioclavicular (AC) Joint

• Articulation between lateral clavicle & acromion of scapula
• Incongruent plane, synovial joint
• Possesses 3 rotational & 3 translational degrees of freedom
• It enables the scapula to move in 3 dimensions during arm movement
  • Increases upper extremity (UE) motion
  • Positions glenoid beneath humeral head
  • Helps maximize scapula contact with thorax
• Assists in force transmission from UE to clavicle

Key features of AC joint

• Variability in shape of articular surfaces from flat to concave/convex
• Relatively vertical orientation of joint surfaces makes it more susceptible to shearing forces, which lead to degenerative effects
• Initially, fibrocartilaginous union between clavicle & acromion
• It develops joint space with Upper extremity (UE) use over time, potentially leaving a "meniscal homologue" within the joint
• Fibrocartilage remnant (disc) varies in size among individuals

Acromioclavicular Capsule and Ligaments

• Capsule of AC joint is relatively weak
• Reinforced by:
  • Superior acromioclavicular ligament
  • Inferior acromioclavicular ligament
  • Coracoclavicular ligaments
• The superior acromioclavicular ligament resists anteriorly directed forces applied to the lateral clavicle and is reinforced by aponeurotic fibers of trapezius & deltoid muscles
• Stronger than inferior capsule and ligament
• Coracoclavicular Ligament divided into:
  • Conoid ligament (more triangular & vertically oriented)
    • Primary restraint to inferior translation of acromion relative to lateral clavicle
  • Trapezoid ligament (quadrilateral and oriented more horizontally)
    • Restraint to posterior translations of lateral clavicle relative to acromion
• The coracoclavicular ligament in both portions limits upward rotation of the scapula at the AC joint and coupling posterior clavicle rotation & scapula upward rotation during UE elevation

Acromioclavicular Joint Kinematics

• Axes of motion are difficult to define: Due to variability in joint surfaces among individuals
• Motions including internal/external rotation, anterior/posterior tilting, and upward/downward rotation occur around axes oriented relative to plane of the scapula
• Superior view of scapula is found is resting in internally rotated position 35° - 45° anterior to coronal (frontal) plane
• Lateral view of scapula is found anteriorly tilted ~10° - 15° from vertical
• The ``longitudinal” axis of the scapula at rest is upwardly rotated 5° - 10° from vertical
• Internal /external rotation (IR/ER) of scapula at AC joint occurs around a nearly vertical axis
• Anterior/posterior (Ant/post) tilting occurs around an oblique "coronal" axis
• Upward/downward rotation occurs around an oblique “A-P” axis

Actions at the Acromioclavicular Joint

• Internal & External Rotation orients glenoid fossa anteromedially and posterolaterally. This maintains contact of scapula with curvature of thorax.
• Positions the glenoid fossa toward plane of humeral elevation
• Maintains congruency & stability between the humeral head & scapula
• Maximizes function of GH muscles, capsule, & ligaments

Anterior and Posterior Tilting motions at the Acromioclavicular Joint

• Occurs around oblique "coronal" axes
• During Anterior tilting the Acromion moves forward & inferior angle moves posteriorly
• During Posterior tilting the acromion moves backward & inferior angle moves anteriorly
• Anterior tilting occurs in combination with scapular elevation
• Posterior tilting occurs in combination with scapular depression

Upward & Downward Rotation at the Acromioclavicular Joint

• Occurs around oblique “A-P” axis
• During Upward Rotation the glenoid fossa tilts upward & inferior angle moves laterally
• During Downward Rotation the glenoid fossa tilts downward & inferior angle moves medially
• Isolated passive motion of upward/downward rotation at AC joint is limited by the coracoclavicular ligament
• Posterior rotation of clavicle reduces tension of coracoclavicular ligaments. This "opens" the AC joint, and allowing upward rotation to occur

Stability of the AC Joint

• Stability of the AC joint is not inherently stable
• Susceptible to trauma & degenerative changes
• Trauma related AC joint dysfunction is more common in the first 3 decades of life.
• Contact sports or a fall on shoulder with the arm adducted can cause AC Joint instability
• Degenerative changes are more common later in life

Scapulothoracic "Joint"

• It is formed by the anterior surface of scapula and the thorax and is not a true anatomic joint
• SC & AC joints are interdependent with scapulothoracic motion: any movement of scapula on thorax must result in movement at AC joint, SC joint, or both
• Stability related to:
• Integrity of the AC joint & SC joint
• Muscle strength and control
• Dynamic stabilization

Scapulothoracic Kinematics

• Scapula rests on posterior thorax ~5 cm from midline between the 2nd – 7th ribs
• It's position is internally rotated, ~35° - 45°, anteriorly tilted, ~10° - 15° ,and upwardly rotated 5° - 10°
• Has rotational movements, including upward/downward rotation, internal/external rotation and anterior/posterior tilting
• Scapulothoracic elevation/depression & protraction/retraction are considered translatory motions

Important kinematic notes

• Upward rotation of the scapula is the principal motion of scapula during active elevation of arm
• Full upward rotation of scapula requires: elevation at the sternoclavicular joint, clavicular posterior rotation, and upward rotation at the AC joint
• Scapulothoracic elevation and Clavicular elevation are key parts of elevation and depression
• Small adjustments at the AC joint for internal rotation (IR)/external rotation (ER) or anterior/posterior (ant/post) tilting maintain contact with thorax
• Full scapular protraction results in an anteriorly facing glenoid
• Normally 15° of IR occurs at the AC joint with regular elevation of the arm
• Excessive IR of scapula on thorax causes increased prominence of medial border of scapula (Scapular winging), potentially indicating pathology or poor neuromuscular control of the scapulothoracic muscles (esp the serratus anterior)
• Primarily occur at the AC joint and couples with rotation of clavicle at the SC joint
• Excessive anterior tilting can result in prominence of inferior angle of the scapula because of because of poor neuromuscular control, faulty posture, and/or muscle tightness (pec minor)

Muscle Actions of the Scapulothoracic Joint

• Scapular protraction: serratus anterior, pectoralis major, and pectoralis minor
• Scapular retraction: middle trapezius Rhomboids
• Scapular elevation: upper trapezius and Levator scapulae Rhomboids
• Scapular Depression: Lower trapezius and Latissimus dorsi, Pectoralis minor
• Scapular Downward Rotation: Rhomboids and Latissimus dorsi, Levator scapulae, Pectoralis minor
• Scapular Upward Rotation: Upper trapezius and Serratus anterior, Lower trapezius

Glenohumeral (GH) Joint

• Ball & socket, synovial joint with 3 rotary & 3 translatory degrees of freedom
• It is defined by articulation between the humeral head and glenoid fossa withscapula influence on GH joint function
• Is designed for mobility, although Reduced stability increases susceptibility to instability, injury and degenerative changes

Key features of the Glenohumeral (GH) Joint

• Articular surfaces
• Glenoid fossa has a shallow concavity
• Orientation of glenoid varies with respect to resting position of scapula and it is often slightly tilted upward
• Fossa is not always in a plane perpendicular to plane of the scapula. It is sometimes Anteverted which means glenoid fossa faces slightly anterior with respect to plane of scapula. In other cases Retroversion has the glenoid fossa facing slightly posterior The glenoid is, most commonly, in slight retroversion (6° - 7°)
• Articular surfaces
  • Humeral head forms 1/3 to 1/2 of a sphere and its articular surface area is larger than that of the glenoid
  • Angle of inclination (Normally between 130° - 150°) is Formed by an axis through humeral head & neck in relation to a longitudinal axis through the humeral shaft
  • Is the angle of torsion that's formed by an axis through the humeral head and neck in relation to an axis through the humeral condyles

Stabilization of the GH Joint

• Normal, which is also slightly retroverted, centers humeral head on glenoid fossa when scapula is in resting position & arm is at side
• Abnormal ,where excessive retroversion or anteversion alters the position of humeral head in the glenoid and potentially predisposes to injury

Glenohumeral Joint: Accessory Structures

• Glenoid labrum surrounds and is attached to glenoid fossa. It enhances articular surface, enhances depth/concavity of fossa by ~50%, resists humeral head translations, protects bony edges of fossa, minimizes GH friction, and dissipates joint contact forces. It is also the attachment site for the long head of the biceps and glenohumeral ligaments

Glenohumeral Capsule & Ligaments

• Large, loose joint capsule that's taut superiorly and loose inferiorly when arm is at rest by the side It maximally tightens when the arm is fully abducted & externally rotated which puts it in the close-packed position. Reinforced by the ligaments
• Superior GH ligament
• Middle GH ligament
• Inferior GH ligament
• Coracohumeral ligament

Glenohumeral Joint Ligaments

• Superior, middle, & inferior GH ligaments are thickened regions within the joint capsule
• Superior GH ligament:
  • Runs from superior glenoid labrum to upper neck of humerus and is deep to the coracohumeral ligament -With rotator interval capsule structures, it limits anterior & inferior translations of humeral head when the arm is at the side
• The middle GH ligament:
  • Runs obliquely from the superior anterior (ant.) labrum to the anterior (ant.) proximal humerus
  • Limits anterior humeral translation with the arm at the side & up to 60° of abduction
• The Inferior glenohumeral ligament complex (IGHLC): 3 components of anterior & posterior ligament bands and an axillary pouch between them
• Has position-dependent variability in function
• The IGHLC function's major role lies with joint stabilization with abd > 45° or with combined abd + rotation where, with ABD > 45°, the inferior capsule slack is taken up and resists inferior humeral head translation
• The rotator interval capsule includes the superior GH capsule, the superior GH ligament, and the coracohumeral ligament
• Bridges gap b/w supraspinatus and subscapularis tendons
• The coracohumeral ligament
  • Originates at base of coracoid process comprised of 2 bands.
  • One inserts into edge of supraspinatus tendon & onto greater tubercle
    • Another 2nd band inserts into subscapularis & lesser tubercle -Forms a tunnel for the tendon of the long head of the biceps, it limits inferior head translation in the dependent position. This resists humeral lateral rotation with the arm abducted

The Subacromial Space

• Known as the suprahumeral space, and bounded by the Coracoacromial Arch:
  • Has undersurface of acromion with coracoacromial ligament and the inferior surface of AC joint
  • Creates an osteoligamentous "vault" over the humeral head
  • The area b/w humeral head and arch is subacromial space
• Contents of subacromial space:
  • Subacromial bursa (reduces friction b/w humeral head and tendons)
  • Rotator cuff tendons
  • Long head of biceps tendon
• Is measured as the acromiohumeral interval on x-rays where healthy individuals have ~10 mm with arm at side and ~5 mm with arm elevated OH

Glenohumeral Kinematics

• Range of Motion:
  • Pure GH flexion is ~ 120°
  • Pure GH extension is ~ 50° Is ROM that varies with position so the GH rotation with arm at side is less than when abducted
• External Rotation (ER) of humerus is needed for full Abduction (ABD) to Allow greater tubercle to pass under or behind coracoacromial arch

Important GH kinematic notes;

• Scaption: the abduction in the scapular plane (30° to 45° anterior to the frontal plane) that Often allows for greater ROM due to less capsular restriction
• Abduction of GH Joint requires an inferior slide of humeral head for normal ROM to occur (ABD arthrokinematics)
• Superior roll of humeral head and Inferior slide of humeral head are critical for healthy abduction
• W/o an inferior slide to counteract the superior roll, the humeral head will impinge on the coracoacromial arch
• With Abduction the articular surface slides inferiorly, & the axis of rotation of humerus shifts slightly superiorly

More on Glenohumeral Arthrokinematics

• Flexion & Extension includes humeral head largely spinning in place During flexion the humeral head spins and with slight posterior slide
• During Extension the humeral head spins with slight anterior slide
• Lateral Rotation -Humeral head rolls posteriorly and slides anteriorly
• Medial Rotation Humeral head rolls anteriorly and slides posteriorly

Stabilization of the Dependent Arm at Rest and Dynamic Stabilization of the Glenohumeral Joint

• The downward pull of gravity is opposed by passive tension in rotator interval capsule to result with resultant force stabilizes humeral head on glenoid fossa
• The important factor of holding the humeral head is the joint capsule having a negative intra-articular pressure as well as slight upward tilt of the glenoid fossa
• Dynamic stabilization is related to: - Force of the prime mover or movers - Force of gravity - Force of the muscular stabilizers - Articular surface geometry - Passive capsuloligamentous forces - Force of friction within joint - Joint reaction force

Functional Muscles

• The important functional result of isolated deltoid would cause more superior translation than rotation of humerus which is overcome by the function of the rotator cuff muscles who majorly contribute to the dynamic stability of the GH joint. The net force of these muscles creates some rotation of humerus and compresses humeral head into glenoid -The muscles of the rotator cuff create an inferiorly directed line of pull those offsets superior translation force of deltoid creating a force couple to produce rotation of humeral head with minimal translaltion`
• Supraspinatus does not contribute to offsetting the the superior translation yet contributes to JT compression.The Relatively large MA allows it to independently produce nearly full GH ABD.Gravity is used a stabilizing synergist those offsets small upward translatory pull of a muscle

Final GH Joint Points

• Muscles of the: supraspinatus, deltoid, subscapularis, infraspinatus all function in the slide ,roll,abduction and rotation motions of the humerous Long head of biceps tendon Appears to center the humeral head in fossa & reduce vertical and anterior translations of humeral head, however The clinical significance here is questionable

Muscle Actions at the GH Joint include:

• Flexion: Deltoid (anterior) Pectoralis major (clavicular head) Biceps brachii, and Coracobrachialis
• Extension: Deltoid (posterior) Latissimus dorsi Triceps brachii (long head) Pectoralis major (sternal head) ,Teres major
• Abduction:Supraspinatus and Deltoid (middle)
• Adduction: Pectoralis major Latissimus dorsi , Teres major,and Coracobrachialis
• Medial (Internal) Rotation: Pectoralis major and Subscapularis as well as Teres major Latissimus dorsi and Deltoid (anterior)
• Lateral (External) Rotation: Infraspinatus and Teres minor as well as Deltoid Posterior

Integrated Function

• Upward rotation of the scapula on the thorax accounts for roughly 50 -60 degrees of motion shoulder elevation while a normal combined motion arc for the shoulder is 180'
• These movements are very coupled together rather than sequentially with at a 2:1 ratio in the shoulder between the humeral head and scapula in the early phases.

In detail with a Glenohumeral Rhythm

• "setting phase" up to ~30° of elevation • Overall ratio of 2° of GH to 1° of ST motion during arm elevation This Ratio varies at different points in the ROM
• Scapular Force Couples are crucial for balance

Scapular Force Couples

• During Upward Rotation :Upper trap, serratus anterior, & lower trap
• During Downward Rotation: Levator scap, rhomboids, & pec minor

Coupled Motions with Glenohumeral Flexion

• In Scapulothoracic Joint: Upward rotation and Posterior tilting which has internal rotation initially, followed by external rotation in higher ranges of flexion
• In Sternoclavicular Joint: Elevation and Posterior rotation and Protraction In Acromioclavicular Joint: Horizontal and sagittal plane rotational adjustments occur with elevation
• In Scapulothoracic Joint: Upward rotation and Posterior tilting with internal rotation initially, is followed by external rotation, in higher ranges of abduction
• In Sternoclavicular Joint: Elevation with Posterior rotation and Retraction
• In Acromioclavicular Joint: Upward rotation with Horizontal and sagittal plane rotational adjustments
• With Lateral Rotaiton and Medial Rotaiton is is coupled as; -Lateral rotation has all its actions in with the Scapultothoracic motion of Retraction as well has Sternoclavicular motion of Retraction -Medial Rotaiton has the reciprocal , Scapultothoracic joint motion of Protraction as well as Sternoclavicular motion of Protraction